This invention relates to a process for cost-effectively producing commercial products from natural gas. More particularly, this invention relates to an integrated process for producing liquefied natural gas and products made through natural gas conversion technology.
Natural gas generally refers to rarefied or gaseous hydrocarbons found in the earth. Non-combustible natural gases occurring in the earth, such as carbon dioxide, helium and nitrogen are generally referred to by their proper chemical names. Often, however, non-combustible gases are found in combination with combustible gases and the mixture is referred to generally as xe2x80x9cnatural gasxe2x80x9d without any attempt to distinguish between combustible and non-combustible gases. See Pruitt, xe2x80x9cMineral Terms-Some Problems in Their Use and Definition,xe2x80x9d Rocky Mt. Min. L. Rev. 1, 16 (1966).
Natural gas is often plentiful in regions where it is uneconomical to develop those reserves due to lack of a local market for the gas or the high cost of processing and transporting the gas to distant markets.
It is common practice to cryogenically liquefy natural gas so as to produce liquefied natural gas (LNG) for storage and transport. A fundamental reason for the liquefaction of natural gas is that liquefaction results in a volume reduction of about {fraction (1/600)}, thereby making it possible to store and transport the liquefied gas in containers at low or even atmospheric pressure. Liquefaction of natural gas is of even greater importance in enabling the transport of gas from a supply source to market where the source and market are separated by great distances and pipeline transport is not practical or economically feasible.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to xe2x88x92240xc2x0 F. to xe2x88x92260xc2x0 F. where it may exist as a liquid at near atmospheric vapor pressure. Various systems exist in the prior art for liquefying natural gas or the like whereby the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages, cooling the gas to successively lower temperatures until liquefaction is achieved. Cooling is generally accomplished by heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, nitrogen and methane, or mixtures thereof. The refrigerants are commonly arranged in a cascaded manner, in order of diminishing refrigerant boiling point.
Additionally, chilled, pressurized natural gas can be expanded to atmospheric pressure by passing the natural gas through one or more expansion stages. During the course of this expansion to atmospheric pressure, the gas is further cooled to a suitable storage or transport temperature by flash vaporizing at least a portion of the already liquefied natural gas. The flashed vapors from the expansion stages are generally collected and recycled for liquefaction or burned to generate power for the LNG manufacturing facility.
LNG projects have not always been economical in that cryogenic refrigeration systems are highly energy intensive and require a substantial capital investment. In addition, participating in the LNG business requires further investment for sophisticated and costly shipping vessels and regasification systems so that the LNG consumer can process the product.
An alternative to the cryogenic liquefaction of natural gas to LNG is the chemical conversion of natural gas into GTL (GTL) products. Methods for producing GTL products can be conveniently categorized as indirect synthesis gas routes or as direct routes. The indirect synthesis gas routes involve the production of synthesis gas comprising hydrogen and carbon dioxide as an intermediate product whereas, for purposes of the present invention, the direct routes shall be construed as covering all others.
Traditional GTL products include, but are not limited to, hydrogen, methanol, acetic acid, olefins, dimethyl ether, dimethoxy methane, polydimethoxy methane, urea, ammonia, fertilizer and Fischer Tropsch reaction products. The Fischer Tropsch reaction produces mostly paraffinic products of varying carbon chain length, useful for producing lower boiling alkanes, naphtha, distillates useful as jet and diesel fuel and furnace oil, and lubricating oil and wax base stocks.
The most common commercial methods for producing synthesis gas are steam-methane reforming, auto-thermal reforming, gas heated reforming, partial oxidation, and combinations thereof.
Steam methane reforming generally reacts steam and natural gas at high temperatures and moderate pressures over a reduced nickel-containing catalyst to produce synthesis gas.
Autothermal reforming generally processes steam, natural gas and oxygen through a specialized burner where only a portion of the methane from the natural gas is combusted. Partial combustion of the natural gas provides the heat necessary to conduct the reforming reactions that will occur over a catalyst bed located in proximity to the burner.
Gas heated reforming consists of two reactors or reaction zones, a gas heated reformer reactor/zone and an autothermal reformer reactor/zone. Steam and natural gas are fed to the gas-heated reformer where a portion of the natural gas reacts, over catalyst, to form synthesis gas. This mixture of unreacted natural gas and synthesis gas is then fed to the autothermal reformer, along with oxygen, where the remaining natural gas is converted to synthesis gas. The hot synthesis gas stream exiting the autothermal reformer is then routed back to the gas reformer to provide the heat of reaction necessary for the gas-heated reformer.
Partial oxidation reforming generally processes steam, natural gas and oxygen through a specialized burner where a substantial portion of the methane is combusted at high temperatures to produce synthesis gas. Contrary to autothermal reforming, no catalyst is present in the partial oxidation reactor.
Current technology for manufacturing synthesis gas is highly capital intensive. Autothermal and partial oxidative synthesis gas methods generally require a costly air separation plant to produce oxygen. Steam reforming, which does not require oxygen manufacture, produces a synthesis gas having a higher hydrogen to carbon monoxide ratio that is less than stoichiometrically optimum for manufacture of Fischer Tropsch products. Additionally, the market for GTL products such as dimethyl ether and Fischer Tropsch products has been volatile or in some cases, insufficiently established to overcome the substantial capital investment risk inherent in erecting such plants.
Natural gas reserve holders have found that substantially increasing the capacity of a LNG or GTL plant can improve plant construction economics. Many of the costs inherent to building such plants are fixed or minimally, do not increase linearly with capacity. However, it has also been found that as more of a single product is produced in a distinct and often isolated geographical region, the product price over cost margin for blocks of product if not all of the plant output is reduced.
Integrating a LNG plant and a GTL plant offers the potential for producing a portfolio of products which can turn projects that would not have been commercially viable for many of the above noted reasons into viable projects. While there have been no integrated LNG/GTL plants built to date, there has been increased interest in combining both technologies at a single plant site.
For example, Geijsel et al., xe2x80x9cSynergies Between LNG and GTL Conversion,xe2x80x9d The 13th International Conference and Exhibition on Liquefied Natural Gas, Seoul, Korea, May 14-17, discloses potential benefits for combining a Fischer Tropsch GTL plant (utilizing a combined partial oxidation/steam reforming synthesis gas preparation step) with LNG manufacture. Geijsel notes several logistical benefits for integrating LNG and GTL including:
sharing the capital costs for infrastructure and general facilities such as marine facilities (harbor and mooring), public roads, telecommunication, fresh water, cooling water, emergency relief, electric power, buildings, and fire fighting systems.
sharing of feed gas preparation, including removal of carbon dioxide.
utilizing low boiling alkanes such as propane recovered from the GTL plant to supplement refrigerant usage on the LNG plant.
integration of heat and power including recovery of surplus energy from the GTL facility to power the LNG facility.
utilizing tail gas from the GTL plant to drive the cryogenic refrigeration compressors at the LNG plant.
utilizing vaporized LNG as a low pressure fuel source to provide heat for the steam methane reforming synthesis gas step.
U.S. Pat. No. 6,248,794 to Gieskes similarly discloses a method for utilizing tail gas from a Fischer Tropsch GTL plant as fuel for a refrigeration plant at an LNG facility.
The above-referenced teachings in the area of integrated LNG with GTL technology are largely directed to the sharing of common plant infrastructure and utilities and other incremental consolidation improvements.
U.K. Pat. Application GB 2357140 to Rummelhoff is directed to a process for integrating natural gas liquids (NGL) recovery, LNG production and methanol manufacture. The Rummelhoff process performs two expansion and separation steps so as to provide energy recovery sufficient to facilitate the separation of higher boiling natural gas liquids (xe2x80x9cNGLsxe2x80x9d) such as ethane and higher boiling point hydrocarbon) from LNG. Subsequent to NGL recovery, the Rummelhoff process provides a single, final stage of expanding and separating so as to remove a natural gas stream from LNG for conveying to post-processing steps such as the production of methanol.
The Rummelhoff process integrates the energy balances of NGL recovery and LNG production while producing an off-gas suitable for processing into methanol which is beyond the aggregative combinations of utility and infrastructure optimization set forth in the prior teachings. However, the integration benefits and improvements described in Rummelhoff are largely realized between the manufacture of NGLs and LNG and result in the availability of a stream for methanol production that is provided under limited process and compositional conditions not ideally suited for GTL manufacture.
The present invention is directed to more effectively integrating the LNG and GTL phases and processing steps of an integrated process.
In particular, it has now been found that performing at least two expansion and separation cycles subsequent to substantial removal of NGLs from a cooled natural gas stream provides substantial integration benefits over processes limited to a single expansion and separation step constrained to processing conditions necessary to produce the final LNG product.
It has also been found that directing an expanded natural gas vapor for GTL conversion, available at more favorable conditions of pressure and temperature, from higher pressure expansion and separation steps, provides substantial energy and capital savings compared to processes which require separate facilities for compressing and heating a natural gas vapor present at near atmospheric pressure and substantially colder temperatures.
It has also been found that performing at least two expansion and separation cycles subsequent to substantial removal of NGLs from a cooled natural gas stream permits the plant operator to customize and improve the quality of the LNG product produced compared to LNG product produced from a single expansion and separation cycle constrained to a final atmospheric LNG separation step.
Therefore, the present invention is directed to an integrated process for producing LNG and GTL products comprising cooling natural gas in at least one cooling step so as to provide a cooled natural gas stream; processing the cooled natural gas stream in at least two expansion/separation cycles, each expansion/separation cycle comprising the Substeps of (a) isentropically or isenthalpically expanding at least a portion of the cooled natural gas steam for producing a natural gas vapor component and a LNG component, (b) separating at least a portion of the natural gas vapor component from the LNG component, and (c) repeating Substeps (a) through (b) wherein at least a portion of the LNG component from the previous expansion/separation cycle is directed to each successive Substep (a) and wherein the final LNG product is the LNG component after the final separating step and is substantially liquid at substantially atmospheric pressure; and converting at least a portion of one or more of the expansion/separation cycle natural gas vapor components into a GTL product.
In another embodiment, the present invention is directed to a LNG product produced by a process comprising the steps of cooling natural gas in at least one cooling step so as to provide a cooled natural gas stream; processing the cooled natural gas stream in at least two expansion/separation cycles, each expansion/separation cycle comprising the Substeps of (a) isentropically or isenthalpically expanding at least a portion of the cooled natural gas steam and producing a natural gas vapor component and a LNG component, (b) separating at least a portion of the natural gas vapor component from the LNG component, and (c) repeating Substeps (a) through (b) wherein at least a portion of the LNG component from the previous expansion/separation cycle is directed to each successive Substep (a) and wherein the final LNG product is the LNG component after the final separating step and is substantially liquid at substantially atmospheric pressure; and converting at least a portion of one or more of the expansion/separation cycle natural gas vapor components into a GTL product.
The fully integrated process of the present invention provides substantial benefits over teachings in the art directed to the sharing of common plant infrastructure and utilities and processes reliant on a single expansion and separation step for producing LNG.
The present invention provides an integrated process for producing LNG and GTL products that effectively shifts non-combustibles such as nitrogen and helium and often residual carbon dioxide from the LNG Phase and LNG product to the GTL Phase and GTL feed where it can be effectively processed.
The present invention provides an integrated process for producing LNG and GTL products that synergistically permits a substantial portion of cooled natural gas vapor or LNG component to be isentropically or isenthalpically expanded and directed to the GTL Phase for conversion to GTL products foregoing the need to recompress and refrigerate such material for reinjection back into the LNG refrigeration system or to reject such stream to fuel. At the same time, the isentropic or isenthalpic expansion autorefrigerates and cools the separated residual LNG component thereby providing a synergistic LNG cooling effect reducing the need for supplementary or external refrigeration.
The present invention provides an integrated process for producing LNG and GTL products that facilitates the production of a LNG product containing a higher total mole percentage of ethane and higher boiling point hydrocarbon and therefore a higher energy content. LNG product having a higher energy content can be of great value in certain geographical markets. As another synergistic benefit to the foregoing, removing ethane and higher boiling point hydrocarbon from the GTL Phase feedstock and incrementally directing this material to LNG product is beneficial in that lower concentrations of ethane and higher boiling point hydrocarbon in the GTL Phase feedstock reduces pre-reforming requirements even to the point of eliminating the pre-reforming step entirely.
The process of the present invention provides an integrated process for producing LNG and GTL products that synergistically and more efficiently utilizes available natural gas pressure while at the same time minimizes compressor capital and/or energy requirements.